1. Technical Field of the Invention
The present invention relates to a method and system for X-ray diagnosis of an object in which an X-ray contrast agent is injected, and in particular, to the method and system preferable to an examination for angiography of a lower limb of the object.
2. Related Art
An X-ray diagnostic system is one of medial imaging modalities that can be utilized for examination and diagnosis for various regions of an object to be examined. One of the examinations carried out by the X-ray diagnostic system is lower-limb angiography of the object.
The lower-limb angiography under the X-ray diagnostic system is carried out such that an X-ray contrast agent is injected into the artery of an object at the groin portion thereof and an X-ray is scanned to track a flow of the contrast agent. Hence the scanning is carried out over a wide range from a region near to the pelvis to the tiptoes. It is difficult to obtain an entire image of such a wide range through one time of imaging, and several times of imaging is carried out to cover such a wide range. The resultant partial images are combined to form the entire image. Since this imaged range contains different parts, such as crural areas, knees, second thighs, and malleolus portions, of which sizes (widths and lengths) are different from each other, halation will occur if, for example, the second thighs are scanned on condition that a range to be X-ray radiated is assigned to a size to be fit to the pelvis. The halation, if occurring, will degrade the quality of images.
To avoid such a drawback, a conventional technique has been provided, which requires that a width-directional opening of an X-ray collimator be adjusted such that an X-ray will not be radiated outside beyond the contour of an object to be scanned.
Another conventional technique for preventing the halation has been known by Japanese Patent Laid-open publication No. 6-217973. The publication (pages 21-22 and FIG. 50) explains that the lower limb is imaged with movements of scan positions, in which a preparation scan (or pre-scan) is first performed to detect contour data of an object to be examined to positional data of a couch on which the object is laid. The detected data of the contours and positions is produced into a control table to be referred when an imaging scan is performed. Specifically, the control table is subjected to reference so that the width-directional opening of an X-ray collimator is controlled for every position of the couch, which prohibits the X-ray from being radiated outside beyond the object's contours. In contrast, in this opening control of the X-ray collimator, the length-directional opening thereof (that is, the opening in a body-axis direction of the object) is always set to a constant value.
However, an actual blood flow speed is not constant over the wide region from the pelvis to the tiptoes. In such a wide region, there are various portions in which the blood flows at slower speeds and the blood flows at faster speeds. Furthermore, such a wide region includes some portions through which blood vessels run in simple and/or complicated ways. Hence if the lower limb is imaged with the movements of scanning positions in the condition in which the longitudinal opening of the X-ray collimator (that is, the opening of the X-ray collimator in a direction along the lower limb of an object) is held constant, the resultant images suffer from having some portions that are insufficient for diagnosis.
To overcome this difficultly, it may be possible to employ a technique in which an imaging interval along the direction of the lower limb of an object is shortened to increase the number of times of imaging. Such a technique forces the longitudinal opening of the X-ray collimator to be narrowed, so that the foregoing difficulty can be improved, but an operator should accept a narrowed display area of scanned images and is obliged to track the flow of the contrast agent for imaging by using the narrower-display-area images. Hence the operations for the imaging are complicated and ballooned.